Recently, a hydrodynamic bearing device is used in a motor for rotationally driving a storage medium such as a hard disk drive for performance purposes such as quietness and impact resistance. Known examples of the basic configuration of this hydrodynamic bearing device include the bearing configuration of a motor described in JP 2006-136180A (laid-open on May 25, 2006).
More specifically, as shown in FIG. 1 of JP 2006-136180A, the hydrodynamic bearing device is mainly constituted by a sleeve serving as a fixed member and a shaft serving as a rotating member that is supported so as to be rotatable inside the sleeve, where a clearance between the members is filled with lubricating oil, thereby forming a radial bearing portion and a thrust bearing portion. More specifically, the radial bearing portion is formed between the outer circumferential face of the shaft and the inner circumferential face of the sleeve, and the thrust bearing portion is formed between one face in the axis direction of a thrust flange that is fixed to one end of the shaft and a sleeve end face, and between the other face in the axial direction of the thrust flange and a thrust plate face that are opposed thereto.
Furthermore, a spindle motor is implemented by fixing a disk mounting hub to the hydrodynamic bearing device, and forming an electromagnetic driving portion constituted by a magnet fixed to the hub and a stator fixed on the sleeve side. Herein, in particular, in a spindle motor for a 2.5 inch or smaller hard disk drive, the shaft is joined to the hub by press-fitting or bonding, and a magnetic disk is mounted on the hub. In order to fix the magnetic disk to the hub, a clamper is attached to the shaft and the hub with a bolt, and the magnetic disk is pressed against a disk mounting face. The bolt is threaded into a tapped hole formed at the other end of the shaft. In such a structure, necessary runout precision (perpendicularity) has to be secured between the hub and the shaft, and the hub and the shaft has to be managed such that they are lower than the clamper and that their runout precision (perpendicularity) is relative to the clamper is good. Thus, as shown in the drawing, the shaft is stepped, and the hub is joined such that the hub is seated on the stepped portion of the shaft.
A first small-diameter projecting portion for fixing the thrust flange is formed on one end side of the shaft. The diameter and the projection amount of the first projecting portion relate to the joint strength between the shaft and the thrust flange, and runout precision between the shaft and the thrust flange. Thus, these items are set so as to obtain satisfactory joint strength and runout precision. Moreover, an end face around the first projecting portion serves as a thrust flange attachment face on which the thrust flange is seated, and thus the perpendicularity of the end face with respect to the main body outer circumferential face is required to be highly precise.
In order to secure the joint strength between the shaft and the thrust flange, it is conceivable, for example, to increase the amount of projection of the first projecting portion in the axis direction. However, since room has to be kept also for the space constituting the radial bearing portion, the amount of projection of the first projecting portion in the axis direction cannot be extremely large. Thus, in order to secure the joint strength between the shaft and the thrust flange, generally, the outer diameter of the first projecting portion is increased to the greatest extent possible.
The outer circumferential face of the shaft defines a part of the radial bearing portion of the hydrodynamic bearing device, and thus it is necessary to precisely process the outer circumferential face of the shaft. Thus, a centerless grinding machine is generally used to grind the outer circumferential face of the shaft in the final finish. A centerless grinding machine has, for example, a blade that extends in one direction, and a grinding roller (rotating at high speed) and a feeding roller (rotating at low speed) that extends in the same direction parallel to each other. The grinding roller and the feeding roller are arranged with a predetermined space interposed therebetween in the path above the blade, and rotate in mutually opposite directions. A plurality of shafts are fed to a position between the rollers on the blade. At that time, the shafts are braked by the feeding roller and the blade, and outer circumferential faces of the shafts are ground by the grinding roller. The shafts are transported in the longitudinal direction because the central axis of the feeding roller is inclined as appropriate. With the above-described operation, the plurality of shafts are transported in one direction while end portions in the axis direction of the shafts abutting against each other.
On the other hand, as described above, the first projecting portion to which the thrust plate is to be attached is formed at one end of the shaft for the hydrodynamic bearing device, and the tapped hole is formed at the other end. Thus, in the centerless grinding machine, the following three cases are conceivable as to how the shafts abut against each other.
(First case) The end portions on the side of the first projecting portions of the shafts abut against each other.
(Second case) The end portions on the side of the tapped holes of the shafts abut against each other.
(Third case) The end portion on the side of the first projecting portion abuts against the end portion on the side of the tapped hole of the shafts.
In the first and the second cases, there is no particular problem, because the end faces abut against each other. However, in the third case, due to the relationship between their sizes, the first projecting portion may interfere with the tapped hole, and thus the shafts may become engaged (that is, caught), so that the centers of their axes may be inclined to each other. In this case, the centerless grinding machine cannot precisely process the outer circumferential faces of the shafts. More specifically, the cylindrically and the roundness on the outer circumferential faces of the shafts are deteriorated. As a result, the perpendicularity of the thrust flange attachment face on the side of the first projecting portion decreases with respect to the outer circumferential face of the shaft, and thus the performance of the hydrodynamic bearing device is deteriorated.
It should be noted that even when the surface roughness of portions that interfere with each other is reduced, if the coaxiality of the tapped hole or the first projecting portion with respect to the outer circumferential face of the shaft is not sufficiently high, then the shafts cannot be prevented from tilting at the time of grinding.
Herein, when feeding the shafts to the centerless grinding machine, it takes effort and increases production cost to feed the shafts with an arrangement such that the above-described third case does not occur. Moreover, even when the shafts are rearranged, loading errors cannot be completely prevented.